ObjectSpace.each_object(Foo).count - ruby-on-rails

Question

I'am trying to figure out ObjectSpace.each_object
In console:
class Foo; end
Foo.new
ObjectSpace.each_object(Foo).count
=> 1
GC.start
ObjectSpace.each_object(Foo).count
=> 1
I've seen examples and I know that the second count should be 0.
Any ideas what is going on here?
Thanks.

Answer 1

It depends on your console.
IRB
The last result is saved as _, even if it hasn't been explicitely assigned.
Running GC.start won't remove the last object :
irb(main):001:0> class Foo; end
=> nil
irb(main):002:0>
irb(main):003:0* Foo.new
=> #<Foo:0x007fca7a309f98>
irb(main):004:0> p ObjectSpace.each_object(Foo).count; GC.start; p ObjectSpace.each_object(Foo).count
1
1
=> 1
irb(main):005:0> p ObjectSpace.each_object(Foo).count; GC.start; p ObjectSpace.each_object(Foo).count
1
0
=> 0
Pry
You can access the last result and the second to last result with _ and __ :
[1] pry(main)> 'a'
=> "a"
[2] pry(main)> 'b'
=> "b"
[3] pry(main)> p _, __
"b"
"a"
=> ["b", "a"]
Pry saves all the 100 last results in _out_ Pry::HistoryArray:
[1] pry(main)> class Foo; end
=> nil
[2] pry(main)> Foo.new
=> #<Foo:0x007fd093102118>
[3] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[4] pry(main)> GC.start
=> nil
[5] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[6] pry(main)> _out_[2]
=> #<Foo:0x007fd093102118>
You can use _out_.pop! to remove its last element :
[1] pry(main)> class Foo; end
=> nil
[2] pry(main)> Foo.new
=> #<Foo:0x007fa90b1ad360>
[3] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[4] pry(main)> GC.start
=> nil
[5] pry(main)> ObjectSpace.each_object(Foo).count
=> 1
[6] pry(main)> 5.times{_out_.pop!}
=> 5
[7] pry(main)> GC.start
=> nil
[8] pry(main)> ObjectSpace.each_object(Foo).count
=> 0
Inside a script
If you execute :
class Foo; end
Foo.new
p ObjectSpace.each_object(Foo).count
GC.start
p ObjectSpace.each_object(Foo).count
inside a script, you get :
1
0

Answer 2

GC.start does not force the garbage collector to start.
It is slightly unclear from the documentation, but it just instructs the engine to schedule a garbage collection. That said, one can not rely on GC.start would immediately remove objects from the heap.